A cellular radio system comprises stationary base stations, each having a particular coverage area, and terminals which can move in relation to the base stations and their coverage areas. The coverage areas are also called cells. In this patent application a mobile phone is treated as an illustrative terminal. When a particular mobile phone is switched on, it somehow tries to find the best received signal of a base station and tries to register with the so called location area (LA) which this base station represents. Registration means that the mobile phone informs the mobile network through the base station that it can receive calls via that location area, to which said base station is associated. In the idle mode a mobile phone regularly receives messages transmitted by the base station in order to detect paging messages, which represent an incoming telephone call, and other messages intended for this mobile phone. At the same time the mobile phone monitors the power of signals transmitted by other adjacent base stations, so that it rapidly can shift base stations, when required.
The idle mode operation of a mobile phone according to the GSM system (Global System for Mobile telecommunications) and its extension the DCS1800 (Digital Communications System at 1800 MHz) is described below, in order to explain the background of the invention. Said functions are described in more detail in the EBU (European Broadcasting Union) and ETSI (European Telecommunications Standards Institute) standards ETS 300 535 (GSM 03.22) and ETS 300 578 (GSM 05.08). To a person skilled in the art it is obvious that as a background of the invention the main part of these studies can be generalised so that they are applicable to all digital cellular radio systems.
There are four requirements on a cell, so that a mobile phone normally can camp within it:    the cell must belong to the network of the selected operator;    the cell must not be barred by the network;    the location area represented by the cell must not be included in the list of forbidden location areas defined for each mobile phone; and    the attenuation on the radio path between the mobile phone and the base station must be lower than a certain threshold value defined by the operator (for short this requirement is called the path loss criterion).
A cell which meets the above listed requirements is called a suitable cell. When a mobile phone is switched on it receives the so called BCCH signals (Broadcast Control CHannel) and runs through them in their order of strength, and begins to operate in a suitable cell with the strongest signal. The BCCH signal can also contain a recommendation value attached to the cell which tells whether the cell is recommended by the system or not. The mobile phone begins to operate in a not recommended cell only if no suitable recommended cells are available. This step is called cell selection.
The mobile phone will regularly check whether there is a suitable cell in the vicinity which is more advantageous regarding the radio communication, and if required the mobile phone will perform cell reselection. The mobile phone can select a new cell for three alternative reasons:    according to particular cell reselection criteria the new cell is better than the current cell;    some characteristics of the current cell change, so that this cell is no longer suitable, but the new cell is suitable; or    the mobile phone detects that the downlink signalling connection is interrupted in the current cell.
Cell selection and cell reselection are based on two parameters calculated by the mobile phone, the so called C1 and C2 parameters defined in the standard ETS 300 578 (GSM 05.08). Of these the first one, the C1 parameter, describes the power level received by the mobile phone from the examined base station, in relation to the minimum value of the received power level defined by the system and the maximum permissible transmission power of the mobile phone. The value of the C2 parameter is influenced by the value of the C1 parameter and two correction factors, of which the first one is an offset parameter transmitted by the base station and the second one is a time delay, which aims at preventing rapid consecutive cell reselections by the mobile phone.
Successful management of radio communication or the optimal routing of connections between the terminals and the base stations has an essential effect on the service level which the radio system is able to provide to the users. Particularly in areas with very dense traffic the cells can be partly or totally overlapping, whereby it is required that the mobile phones and other terminals can be controlled to use particular cells and avoid particular other cells in order to guarantee a uniform service level. As an example we could consider an office building which is located within the coverage area of a public cellular radio system, but which also has an internal wireless communication system operating as an extension to the public system, whereby the wireless system is based on so called nano cells or pico cells having a size of one room or a few rooms. For a mobile phone belonging to an employee working in the building it is often more advantageous to operate in a cell of the building's internal system than in a cell of the public cellular radio system. The operator managing the cellular radio system can for each mobile phone also define a so called home area comprising a single cell or a few cells of the public cellular radio system, where the mobile phone is offered cheaper tariffs or other benefits in the home area. On the other hand it is advantageous to define some cells as handover cells only, whereby it is desired that no mobile phones operate in such a cell for a longer period than required by the handover function.
In a system according to the prior art described above there are no possibilities to realise priority cells relating to individual mobile phones. The first correction factor or the offset parameter associated with the C2 parameter's definition can be used for general prioritising, so that a certain value of the offset parameter transmitted by a base station causes all mobile phones to generate a C2 parameter value indicating a disadvantageous cell selection. However, a prioritising of this type does not function differently for each mobile phone, but it is identical for all mobile phones.
From the patent publication U.S. Pat. No. 4,916,728 (Blair) a practice is known in which a mobile phone can operate in networks managed by several different operators. In order to be able to select the network of the most advantageous operator the mobile phone goes through several receive frequencies, decodes the SID codes (System IDentification) from the signals transmitted by the base stations, and tunes to that frequency on which the received SID code indicates the most advantageous operator. The information about the advantages of different operators is stored in the memory of the mobile phones, so in this arrangement different mobile phones react differently on the information transmitted by the base stations. However, in this method it is not possible to have the mobile phones to function differently, except for the selection of the operator, because all base stations in the network of a certain operator transmit the same SID code.
The PCT application publication WO 95/24809 (Motorola Inc.) treats a system in which the central equipment checks, based on the identity transmitted by the mobile station, whether this mobile station is authorised to a certain service in a particular area. If particular regional restrictions and/or restrictions relating to individual mobile phones are defined for the service, then the central equipment can either refuse to provide any services to a particular mobile phone in said region, or allow the use of only one service, e.g. data communications. However, in order to change the offered services the mobile phone must move, because the restrictions are always the same in a particular region. Thus in this method it is not possible to influence the cell selection or cell reselection when the mobile phone or another terminal of the cellular radio system is stationary.
From the Finnish patent application no. FI 952965 and the corresponding European patent publication no. EP 749 254 A1 (Nokia Mobile Phones Oy) there is known a multi-level home area pricing for a mobile phone of a cellular radio system, in which a certain binary character string is stored in the mobile phone. Then each base station transmits its own binary identity at regular intervals and the mobile phone uses the binary character string stored in it as a mask, with which it selects particular bits from the character string transmitted by the base station as the object for a logical comparison operation. If said logical comparison operation generates the correct result the mobile phone construes itself to be in the home area or in another area where a particular regional service is available. Using different logical comparison operations it is possible to form a number of individual areas, or areas located in a mutual hierarchy in which the mobile phone can obtain different services from the cellular radio system. Even this practice is not applicable for proper cell prioritising, because the services are regional and the offered services change only when the mobile phone is moving.
In addition to the above mentioned known methods there are a number of known methods and systems in which a mobile phone or another terminal of a cellular radio system can detect whether or not it operates in a priority cell associated to this device, and provide information about this to the user. However, in any of these methods a user or a terminal is not able to contribute to the cell selection and to decide whether a certain cell selection is retained even when the terminal is stationary.